The present invention relates to a process for preparing an optically active .gamma.-halo-.beta.-hydroxybutyric acid ester by microbiologically treating a .gamma.-hyloacetoacetic acid ester.
In case of preparing a .gamma.-halo-.beta.-hydroxybutyric acid ester by chemically reducing a .gamma.-haloacetoacetic acid ester, a yield of desired products is low because dehalogenation occurs easily.
On the other hand, side reactions such as dehalogenation do not occur in a process for microbiologically reducing a .gamma.-haloacetoacetic acid ester. There is known a process for microbiologically preparing a .gamma.-halo-.beta.-hydroxybutyric acid ester by employing a baker's yeast under an aerobic condition. But the process gives only a low yield of the desired products, which is not practical on an industrial scale.